Acoustic transducer with impedance matching layer

ABSTRACT

A piezoelectric transducer is mounted in a housing. An impedance matching layer is provided between the piezoelectric transducer and the housing. The impedance matching layer includes high-permittivity regions that have a first permittivity, and low-permittivity regions that separate the high-permittivity regions and have a second permittivity. The second permittivity is lower than the first permittivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/984,716, filed on Aug. 8, 2013, entitled “Acoustic Transducer withImpedance Matching Layer,” which is the United States National Stage ofInternational Application No. PCT/US2011/24841, filed on Feb. 15, 2011,entitled “Acoustic Transducer with Impedance Matching Layer.”

BACKGROUND

Acoustic transducers are used for a variety of purposes, including inthe oil field. A focused planar transducer, such as that described inU.S. Pat. No. 5,044,462, which is owned by the assignee of the instantapplication, allow acoustic illumination with focus and depth of field.Transferring energy from an acoustic transducer to fluids in a boreholeis a challenge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view through an acoustic measuring tool inaccordance with the teachings of the present disclosure suspended in awell borehole and which incorporates a rotatable transducer antenna forsending and receiving acoustic pulses against the sidewall of theborehole

FIGS. 2A and 2B are enlarged views of the acoustic transmitter elementhaving several grooves cut therein to define concentric rings fortransmitting purposes.

FIGS. 3A and 3B are an alternate form of an acoustic transmitter elementutilizing square cuts in the ceramic member showing the square elementsconnected in the manner shown in FIG. 3C to closely approximate therings of the transmitter element of FIG. 2.

FIG. 4 is an electronic schematic block diagram showing the componentsof the transducer system and selected waveforms.

FIG. 5 is a block diagram showing the implementation of the phasecontrast imaging process using digitized data and also shows a gaincurve and a processing flow chart.

FIG. 6 is a graph showing beam widths as a function of range and chosenfocusing distance.

FIG. 7 is a graph showing a comparison between amplitude, travel time,and phase contrast processing of data.

FIG. 8 is a sectional view through an acoustic transmitter elementhaving an acoustic impedance matching layer.

FIG. 9A is a sectional view through an acoustic transmitter elementhaving an acoustic impedance matching layer, having slots cut throughthe acoustic impedance matching layer and into a piezoelectric ceramicdisk. FIG. 9B is a plan view of the acoustic transmitter element of FIG.9A.

FIG. 10 shows a transmitted signal without an acoustic impedancematching layer.

FIG. 11 shows a transmitted signal with an acoustic impedance matchinglayer.

FIG. 12 is a block diagram of an environment.

DETAILED DESCRIPTION

Attention is directed now to FIG. 1 of the drawings where the numeral 10identifies an acoustic measuring device supported in a sonde 12 inaccordance with the teachings of the present disclosure. The sondeencloses a telemetry system 14 which provides an output signal on alogging cable 16 which extends to the surface. The sonde includes anantenna rotator 18 for rotating an antenna or transducer 20 inaccordance with the present disclosure. It will be described in greaterdetail hereinafter. The antenna is mounted on a rotatable mechanism 22so that the emitted acoustic pulse travels radially outwardly along apropagation line 24 and impinges on the sidewall 26 of the borehole. Thesonde 12 is constructed with a housing 28 which is elongate andcylindrical. The antenna 20 is preferably submerged in the boreholefluid 30 to provide better acoustic coupling and reduced beam divergencethan that offered by enclosing the transducer in an oil bath andutilizing a window for the acoustic energy to pass through.

As will be understood, the well borehole 26 has been represented as arelatively smooth surface. In fact, it is not and it can be irregulardepending on the nature of the drilling process and the nature of theformations penetrated by the borehole 26.

The conductor 16 extends to the surface where it passes over a sheave38. The sheave 38 directs the logging cable 16 to a drum 40 where it isspooled for storage. The conductors in the cable 16 are connected withsurface located electronics 42. The output data is displayed on adisplay 44. The data is recorded electronically 48, simultaneously withdepth and time. The time is obtained from a real time clock 52 withmillisecond resolution. The depth is provided by an electrical ormechanical depth measuring apparatus 46 which is connected with thesheave 38 and which also connects to the recorder 48. The presentapparatus further includes acoustic electronics 50 which are supportedin the sonde. The acoustic logging device 10 will be understood onexplanation of the various components discussed below.

The antenna 20 is better identified in FIGS. 2A and 2B of the drawingswhere it is shown to be a circular piezoelectric disk member 64. It ispoled in the thickness mode, typically having both flat surfaces coatedwith a conducting metal electrode. It has a solid backing 70 which actsas a highly attenuative medium absorbing the acoustic energy which isradiated into it. The ceramic and backing are housed in an epoxymaterial 68 having a thickness separating the ceramic from the boreholefluid by a quarter wavelength. This material 68, having an intermediateacoustic impedance, is a well known technique for improving the transferof acoustic energy from the ceramic which has a high impedance to thewater (mud) which has a lower impedance. The ceramic is cut with aplurality of circular grooves at 52 and 54. These grooves typically donot fully penetrate the ceramic device for ease of manufacturing.Rather, they define ring shaped surface areas and provide acoustic aswell as electrical isolation between the individual elements. Inside thesmallest ring shaped surface 58 is the center disk 56. This patterncontinues to the outer ring shaped area 60. Electrical attachments aremade to the ceramic using solder or conductive epoxy. The groundelectrode 66 is attached before the ceramic is bonded to the backingmaterial. Each of the rings is attached at 62 to conductors 72 usingeither solder or conductive epoxy. The wires, now attached to thevarious rings, and a single ground electrode are preferably led to theback of the transducer, being held in place by the surrounding epoxyhousing 68. The total number of ring shaped areas is N where N is awhole number integer. Moreover, N is typically in the range of aboutthree at the low end, and increases up to about ten. In theory, N canincrease further, but there is a practical limit in the benefit obtainedby increasing N. The optimum number for N is about three to eight. Inthis particular embodiment, the rings are not evenly spaced radially butare spaced based upon a more subtle criterion. They are spaced such thatwhen focused at the minimum range, the difference in time delay requiredfor each ring is a fixed value. This procedure simplifies theelectronics components somewhat. The delay differences for all of theranges may be kept the same simplifying the electronic design. Even ifrespective delays are slightly imperfect, there is little degradation inthe resulting focusing. There are other methodologies which may be usedto select the spacings of the rings.

FIGS. 3A and 3B show an alternate method for defining the ring shapedpattern on the ceramic element 64. The ceramic 64 is cut into squareelements 74. The individual elements are then connected to form a set ofinterconnected areas simulating a ring shaped area. One example is asshown where the elements all labeled 0 are connected together. Likewise,all elements labeled 1 are connected, and so on, through the elementslabeled 5. This method of construction has several advantages over thesimple ring configuration. The straight lines are easier to cut usingstandard production tools. In the previous design, each of the rings hasa slightly different resonant frequency because their geometries areeach slightly different. The differences in frequency, slightly reducethe imaging resolution of the transducer. The cuts are again 90 percentof the way through the solid ceramic body and are preferably less than0.6 times the thickness of the ceramic in spacing. The electrodes of theindividual square elements 74 are connected in FIG. 3C using small beadsof silver epoxy, 76 to connect the correct pattern of square surfaces.Where a diagonal connection is required, a wire 78 is placed across thediagonal and silver epoxy 80 is used to bond it to the square element 74and hold it above any elements it crosses without connection. The wiresto the electronics are attached as shown in FIG. 2. The resonantfrequency of each square element is the same but slightly lower than thebasic thickness resonance of the disk. The result is that each ringformed by the set of squares has the same resonant frequency andmechanical Q. Each of the rings therefore behaves almost identically intheir source and receiver characteristics.

Each of the rings 56, 58 and 60 shown in FIG. 2A ranging from thesmallest on the inside to the largest on the exterior is used as aseparate transmitting transducer. They are connected to their owndedicated transmitter and receiver units. This is better shown on reviewof FIG. 4 of the drawing where the electronics is shown. First of all,the electronics in FIG. 4 includes N duplicate circuits. Thus, if thereare six rings in the acoustic transducer assembly, then six duplicatecircuits are provided. The description set forth hereinbelow cantherefore be extended to all of the N circuits. The acoustic electronics50 incorporates range select logic 90 which determines the focaldistance of the transducer, both for the transmit mode and the receivemode. The transmit focus may be controlled independently from thereceive focus. The transmit focal distance is sent to the timing driverlogic 82 which controls the N signals going to each of the N transmittercircuits 84. In its simplest format the transmit pulse is delayed by thedecrease in travel time required for the acoustic energy to propagatefrom each ring to the desired focal depth as the ring diameterdecreases. The outer ring typically has no delay, and the inner disk hasthe most delay. The signal out of the transmitter circuit 84 may beeither a single pulse or a burst (typically a square wave) signal at theresonant frequency of the transducer. The N transmit-receive switches 94are used to protect the N preamp circuits from the high voltage transmitpulse on the ceramic. The preamps 86 have typically 20 dB of gain to getthe signal level up to a suitable level and have a lower outputimpedance than the ceramic, allowing them to drive the delay lines 88.The N multiple tap delay lines are used to compensate for the traveltime differences of the signal at some focal distance propagating toeach ring of the transducer. Again the signal from the center disk willbe delayed the most since it will be the closest to the focus, and theouter ring signal will be delayed the least since it is the farthestfrom the focus. As the focal distance increases, the total range ofdelays decreases. The outputs of the taps of the delay line go into Nsets of analog select gates 92. Although an arbitrarily large number oftaps may be used, a number of from 3 to 10 is sufficient. This givesfrom 3 to 10 discrete focal distances for the transducer. The tapselection and thusly the receiver focal distance is controlled by therange select logic 90. The delay taps are thus selected such that the Nsignals coming from a chosen focal distance all appear at the outputs ofthe N analog select gates simultaneously. The N signals are summed inthe summing amplifier 98 to produce the focused signal output 102. Asecond output 104 is also made available which is the signal from onlythe center element. The peak of the envelope of the signal 102 forms theamplitude signal. The time location of the onset of this signal is usedto derive the travel time, indicating the range to the borehole wall.This forms the typical output signal provided to the surface through thetelemetry so that the system presents an image of what is seen by theequipment in the borehole. The signal processing this signal 102 hasbeen described in previous works.

The phase contrast imaging utilizes the signals 102 and 104. While thesignal 102 represents the highly resolved beam and the signal 104represents a less resolved beam, FIG. 5 shows one implementation of thephase contrast imaging process. The gain graph shows the spatialresolution of the two signals 102 and 104, where the curve 106 is theresolution of 102 and the curve 108 is the resolution of 104. In thisprocessing method, both signals are digitized at about 20 times theresonant frequency of the transducer by the digitizers 112 and 114 andthe data are stored in the memory 116. The two signals are thenprocessed by a digital signal processor 118 to find the differences inarrival times. The signal processing sequence begins at the step 120where the location of the peak of the high resolution signal is found.The phase of both signals is then computed in the step 122, beginning intime where the envelope of the signal becomes greater than one-half thepeak value until it drops in amplitude to below one-half the peak. Theaverage phase difference over the measurement period is then computed inthe step 124. These measurements may be filtered to pass high frequencycontent in the step 126 over the period of one revolution to remove theeffects of the changes in the travel time to the borehole wall becauseof ellipticity or eccentering of the tool. The phase contrast data 128may now be sent up hole and displayed as a gray scale image in place ofthe usual travel time image.

The presentation is typically presented as a function of depth in thewell and as azimuth. For instance, a particular image might be obtainedwherein the depth is 8,000 feet, and the image is positioned so that itis centered on an acoustic propagation line which has an azimuth of zerodegrees or north. Benefits of the present system can be obtained onreview of the graphs. FIG. 6 shows the 3 dB beam widths for three focaldistances of a 2.5 inch diameter transducer operating at 280 kHz. Whenthe transducer is focused at 2.4 inches, the three dB beam width is 0.22inches according to the curve 130. As the distance to the targetchanges, the beam width increases significantly, as would the case befor a fixed focusing transducer. If for instance the range increases to4 inches, the fixed focus transducer would have a beam width of 0.55inches. The electronically focused transducer could be refocused andproduce a beam-width of 0.31 inches, see the curve 132. From this, itmay be seen that the depth of field is relatively small when thetransducer is focused at a short distance. This would indicate that asingle fixed focus transducer would only work well when the distance tothe borehole wall was close to the focal distance of the transducer.With the electronically focused transducer, the focal distance can bechanged to match the variations in the range to the sidewall with whatever precision is deemed suitable. It may be seen that, as the rangeincreases to 6 inches, the focusing does not need to be changed by adistance shown in the curve 134, or about 10 inches as illustrated.

Several processed signals are shown in FIG. 7. The amplitude 136 of theecho from a simulated formation shows many small features. Severaldecreases in the signal amplitude are shown which are fromirregularities in the surface. The travel time data 138 shows some ofthis information. Several of the events shown on the amplitude data arenot visible in the travel time data. The travel time image is not aswell resolved as the amplitude image. The phase contrast image data 140shows as much information as the amplitude data 136. In the past, littlecorrelation could be made between travel time and amplitude data becauseof the differences in resolution. Since they now have the sameresolution, good correlations can be made. Each event in the amplitudedata may now be correlated with the phase contrast data so that changesin acoustic impedance may be separated from changes in the surfacecharacter.

The illustrated system is able to provide better and more detailedobservation of events occurring in the borehole in that it presentssurface detail data not otherwise obtainable heretofore. This enhancedsensitivity enables the device to present data with improved resolutionand sensitivity. Moreover, the depth of field is enhanced. Further, thesensitivity of the system to variations in range is reduced, therebyenabling the device to operate over a wider dynamic range so that thegreater irregularities in the borehole 26 can be observed. Dynamicfocusing correction is markedly improved over the fixed focus systemexemplified in UK patent 2,168,569A. The surface character measurementhas also been enhanced.

In one embodiment, illustrated in FIG. 8, an impedance matching layer isadded to the antenna (or “acoustic transducer” or “transducer”) 805. Inone embodiment, the acoustic transducer components are housed within aglass filled polyether ether keytone (“PEEK”) housing 810. PEEK providesmechanical strength for the downhole drilling environment and has lowloss acoustic properties. In one embodiment, an epoxy or anothermaterial having acoustic properties that are intermediate between theimpedance of the ceramic and the borehole fluid is used to construct thehousing 810. In one embodiment, as mentioned above, the housing 810provides one layer of acoustic impedance matching.

In one embodiment, a piezoelectric ceramic disk 815 provides the activeelement and is cut with slots as shown in FIGS. 2A and 2B, or intosquare elements 74 as shown in FIGS. 3A and 3B. In FIG. 8, thepiezoelectric ceramic disc 815 is cut with slots 820, 825, 830. Theportions of the piezoelectric ceramic disk 815 between the slots 820,825, 830 and the small squares shown in FIGS. 3A and 3B are elements ofa phased array that can be used to focus acoustic signals emitted by thepiezoelectric ceramic disk 815 as described above with respect to FIGS.4 and 5.

In one embodiment, an impedance matching layer 835 is provided betweenthe piezoelectric ceramic disk 815 and the housing 810. In oneembodiment, the purpose of the impedance matching layer 835 is toprovide a better match between the acoustic impedance of thepiezoelectric ceramic disk 815, which is typically high, and theacoustic impedance of a fluid in a borehole, such as drilling mud orformation fluid, which is typically lower than that of the piezoelectricceramic disk 815. In one embodiment, the housing 810 provides anadditional impedance matching layer which means that the acoustictransducer 805 has two impedance matching layers. In one embodiment:

AIT>AI1>AI2>AIBF

where:

-   -   AIT is the acoustic impedance of the transducer; i.e., the        acoustic impedance of the piezoelectric ceramic disk 815;    -   AI1 is the acoustic impedance of the impedance matching layer        835;    -   AI2 is the acoustic impedance of the housing 810; and    -   AIBF is the acoustic impedance of the borehole fluid (e.g.,        borehole fluid 30 shown in FIG. 1).

In one embodiment, additional impedance matching layers (not shown) areprovided between the piezoelectric ceramic disk 815 and the housing 810.In one embodiment, the acoustic impedance of the impedance matchinglayers generally increase the closer they are to the piezoelectricceramic disk 815, although some deviation from this is envisioned. Thatis, the scope of the claims is intended to cover embodiments in which asmall number of impedance matching layers relative to the total numberof impedance matching layers do not strictly follow the monotonic changein acoustic impedance from the piezoelectric ceramic disk 815 to thehousing.

In one embodiment, the impedance matching layer 835 is made of one partDURALCO® 4460 epoxy mixed with 68 parts hematite by weight. The particlesize of the hematite is typical of that used in oil based drilling mud.In one embodiment, the mixture is exposed to a vacuum until no bubblesare present in the mixture and molded onto a flat TEFLON® surface to aninitial depth of 3 times the final thickness. In one embodiment, theepoxy is cured at 180° F. for 4 hours, then at 220° F. for 4 hours. Inone embodiment, the lower density epoxy at the top is removed until thethickness is ¼ wavelength at the center frequency of the transducer. Inone embodiment, the resulting material in this process has an acousticimpedance of 8.2 MRayls a density of 2.82 g/cm³ and a sound velocity of2900 m/sec. In one embodiment, the surfaces of the resulting disk arefinished flat and smooth and placed between the ceramic crystal and thePEEK housing.

In one embodiment, the piezoelectric ceramic disk 815 is supported by abacking material 840 that also absorbs acoustic energy traveling fromthe piezoelectric ceramic disk 815 away from the impedance matchinglayer 835. In one embodiment, an epoxy casting resin 845 is used toclose the housing and provide strain relief for the electricalconductors 850 attached to the piezoelectric ceramic disk 815. Theconnection of the electrical conductors 850 to the piezoelectric ceramicdisk 815 is not shown in FIG. 8 but, in one embodiment, is similar tothat shown in FIG. 2B.

In one embodiment, illustrated in FIGS. 9A and 9B, rings 905, 910, 915,920 are formed by cutting slots 925, 930, 935 through an impedancematching layer 940 and partially through the piezoelectric ceramic disk815. In one embodiment, the slots 925, 930, 935 form the fourindependent active sources 905, 910, 915, 920 for the focusing of theresulting acoustic transducer 945. The PEEK housing 810 is not cutbecause of the requirement that it protect the interior materials fromthe borehole fluid and abrasive materials contained therein.

In one embodiment using the ring pattern illustrated in FIGS. 3A, 3B,and 3C, the impedance matching layer 940 is cut in the same way asillustrated in FIG. 3A.

In one embodiment, the cuts 925, 930, 935 are only partially through theimpedance matching layer 940. In one embodiment, the partial cuts arethrough the side closest to the housing 810. In one embodiment, thepartial cuts are through the side closest to the piezoelectric crystaldisk 815.

In one embodiment, the impedance matching layer 940 has N independentimpedance matching layer regions that match and overlay the Nindependent transducer regions. That is, the four rings 905, 910, 915,920 illustrated in FIG. 9B include rings cut in the piezoelectricceramic disk 815 and matching and overlaying rings cut in the impedancematching layer 940.

In one embodiment, in which the piezoelectric ceramic disk 815 is cut asshown in FIG. 3A and connected into regions (e.g., regions 1, 2, 3, 4,and 5) illustrated in FIG. 3A using the techniques shown in FIG. 3C, theregions in the impedance matching layer 940 match and cover the smallsquares, e.g. square 74, but are not connected together to form theregions (e.g., regions 1, 2, 3, 4, and 5). In one embodiment, thepiezoelectric ceramic disk 815 is cut as shown in FIG. 3A and connectedinto regions (e.g., regions 1, 2, 3, 4, and 5) illustrated in FIG. 3Ausing the techniques shown in FIG. 3C, and the impedance matching layer940 is then applied and slots are cut into the impedance matching layer940 to match the regions illustrated in FIG. 3A.

In one embodiment, the focus distance of the transducer 945, with anactive surface diameter of 1.25 inches and a focus distance of 0.56inches has an outer ring 920 width of 0.734 inches. The thickness of thering when manufactured to be ¼ wavelength thick at 350 kHz will be0.0878 inches. In this embodiment, the sheer coupling between adjacentsegments will be significant. The shear loading will significantlyaffect the energy coupled into the face of the transducer and eventuallyinto the volume to be measured. In one embodiment, a polymer with lowshear stiffness or a fluid is inserted between adjacent phased-arrayelements of the piezoelectric ceramic disc 815, which improves theoperating characteristics compared to a continuous layer. The process ofcutting the intermediate layer to match the ceramic active areas allowsthe use of ceramic materials, which exhibit a smaller change incharacteristics with temperature and lower acoustic loss than weightedpolymers.

In one embodiment, the slots 925, 930, 935 improve the performance ofthe impedance matching layer 940 by eliminating the shear couplingbetween segments or rings of the impedance matching layer 940 that arenot moving in the same direction. In one embodiment, the loss in signalamplitude due to unwanted shear loading is reduced.

In one embodiment, when the transducer 945 is focused at short range,0.56 inches, for example, the phase difference between signals onadjacent rings (e.g., rings 905 and 910) will be 150 degrees. Adjacentrings will be moving in almost opposite directions.

In one embodiment, cutting the rings in the impedance matching layer 940to reduce the coupling between the phased array elements is independentof how the ring electrodes are formed in the crystal. In other words, itis sometimes not necessary to cut groves in the crystal to create theringed electrodes because of the properties of the crystal. That is, inone embodiment, the benefits of the impedance matching layer 940 are notdependent on how the phased array is formed in the crystal.

The desire is to improve the acoustic impedance match as the acousticwave propagates through the matching layers towards the intended targetwhile maintaining its phase integrity. Reducing the shear coupling inthe matching layer enhances focusing and signal-to-noise performance. Inaddition, the slots improve electric isolation between the phased arrayelements. The undesired coupling between electrodes iselectromechanical. The dominant coupling component occurs in thecrystal; however, the electrodes also produce an electric field betweenthe electrodes at the surface of the crystal. The capacitance couplingthat result from this electric field component is dependent on thepermittivity of the selected acoustic matching layer 940 material. Thefield intensity falls quickly with distance from the crystal face. Thecuts in the acoustic matching layer 940 effectively introduce a serieselement, with lower permittivity, that impedes displacement current. Theburden on the drive electronics is reduced and better isolation results.

In one embodiment, the regions of the impedance matching layer 940 havethe same acoustic impedance. In one embodiment, at least one of theregions of the impedance matching layer 940 has a different acousticimpedance than another of the regions of the impedance matching layer940. For example, in one embodiment, the impedance matching layer 940 isformed using a protective face (e.g., PEEK) on the piezoelectric ceramicdisk 815 with non-uniform thickness providing different matching layerimpedances for the array elements.

In one embodiment, cutting slots into the impedance matching layer 940significantly reduces the shear coupling between each signal path untilthe signal reaches the PEEK. In one embodiment, the PEEK is not cut towithstand the harsh borehole environment and the loss is tolerated.

FIGS. 10 and 11 show the difference in signal amplitude transmittedeffected by changing from one layer of PEEK with optimum thickness (FIG.10) to a layer of PEEK and an optimized impedance matching layer 940(FIG. 11). As can be seen, the use of the extra impedance matching layerincreases the signal amplitude by 2.5 dB for transmit signal. Theoverall improvement is 5 dB since the received signal also experiencesthe improvement. The sensitivity of the transducer is almost doubled bythe addition of the impedance matching layer 940.

In one embodiment, the transducers 805 and 945 are used inmeasurement-while-drilling/logging-while-drilling (“MWD/LWD”) systems,wired drillpipe systems, coiled tubing systems (wired and unwired), andwireline systems. In one embodiment of an MWD/LWD system usingelectronics such as that illustrated in FIG. 5 located downhole, theelectronics are powered by an onboard battery or by a generator, such asa mud generator. In one embodiment, power is provided from the surface.In one embodiment, all of the processing discussed above, such as thatassociated with FIG. 5 is done downhole. In one embodiment, theprocessing is done on the surface, for example by electronics 42 in FIG.1.

In one embodiment, the controller for the electronics shown in FIG. 5 isstored in the form of a computer program on a computer readable media1205, such as a CD or DVD, as shown in FIG. 12. In one embodiment acomputer 1210, which may be the same as electronics 42 or acousticelectronics 50 (see FIG. 1), or which may be below the surface in thedrill string, reads the computer program from the computer readablemedia 1205 through an input/output device 1215 and stores it in a memory1220 where it is prepared for execution through compiling and linking,if necessary, and then executed. In one embodiment, the system acceptsinputs through an input/output device 1215, such as a keyboard, andprovides outputs through an input/output device 1215, such as a monitoror printer. In one embodiment, the system stores the results ofcalculations in memory 1220 or modifies such calculations that alreadyexist in memory 1220.

In one embodiment, the results of calculations that reside in memory1220 are made available through a network 1225 to a remote real timeoperating center 1230. In one embodiment, the remote real time operatingcenter 1230 makes the results of calculations available through anetwork 1235 to help in the planning of oil wells 1240 or in thedrilling of oil wells 1240.

The text above describes one or more specific embodiments of a broaderinvention. The invention also is carried out in a variety of alternateembodiments and thus is not limited to those described here. Theforegoing description of the preferred embodiment of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Many modifications and variations are possible in lightof the above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A device comprising: a piezoelectric transducer,the transducer having N independent transducer regions wherein N is aninteger, each of the N independent transducer regions having athickness, each of the N independent transducer regions having anacoustic impedance AIT, each of the N independent transducer regionsbeing independently excitable to oscillate in the thickness mode whenelectrically excited by a potential difference applied across thethickness; a first impedance matching layer having an acoustic impedanceAI1 between AIT and a borehole fluid acoustic impedance AIBF, the firstimpedance matching layer being situated such that an acoustic signalemitted by the piezoelectric transducer will pass through the firstimpedance matching layer, the first impedance matching layer comprising:high-permittivity regions that match and overlay the N independenttransducer regions and have a first permittivity, and low-permittivityregions that separate the high-permittivity regions and have a secondpermittivity wherein the second permittivity is lower than the firstpermittivity; and a second impedance matching layer having an acousticimpedance AI2 between AI1 and AIBF, the second impedance matching layerbeing situated in the device such that an acoustic signal emitted by thetransducer will pass through the second impedance matching layer afterit passes through the first impedance matching layer; wherein the firstimpedance matching layer has N independent first impedance matchinglayer regions that match and overlay the N independent transducerregions.
 2. The device of claim 1 wherein the low-permittivity regionsare formed by cuts in the first impendence matching layer.
 3. The deviceof claim 1 wherein the low-permittivity regions impede displacementcurrents.
 4. The device of claim 1 wherein the low-permittivity regionsact as series elements between the high-permittivity regions.
 5. Thedevice of claim 1 wherein the low-permittivity regions cause the deviceto present a smaller load than without the low-permittivity regions. 6.A method comprising: emitting an acoustic pulse from a piezoelectrictransducer, the transducer having N independent transducer regionswherein N is an integer, each of the N independent transducer regionshaving a thickness, each of the N independent transducer regions havingan acoustic impedance AIT, each of the N independent transducer regionsbeing independently excitable to oscillate in the thickness mode whenelectrically excited by a potential difference applied across thethickness; passing the acoustic pulse through a first impedance matchinglayer having an acoustic impedance AI1 between AIT and a borehole fluidacoustic impedance AIBF, the first impedance matching layer comprising:high-permittivity regions that match and overlay the N independenttransducer regions and have a first permittivity, and low-permittivityregions that separate the high-permittivity regions and have a secondpermittivity wherein the second permittivity is lower than the firstpermittivity; and passing the acoustic pulse that has passed through thefirst impedance matching layer through a second impedance matching layerhaving an acoustic impedance AI2 between AI1 and AIBF.
 7. The method ofclaim 6 wherein the low-permittivity regions are formed by cuts in thefirst impendence matching layer.
 8. The method of claim 6 wherein thelow-permittivity regions impede displacement currents.
 9. The method ofclaim 6 wherein the low-permittivity regions act as series elementsbetween the high-permittivity regions.
 10. The method of claim 6 whereinthe low-permittivity regions cause the device to present a smaller loadthan without the low-permittivity regions.
 11. An apparatus comprising:a housing; a piezoelectric transducer mounted in the housing; and animpedance matching layer provided between the piezoelectric transducerand the housing, the impedance matching layer comprising:high-permittivity regions that have a first permittivity, andlow-permittivity regions that separate the high-permittivity regions andhave a second permittivity wherein the second permittivity is lower thanthe first permittivity.
 12. The device of claim 11 wherein thelow-permittivity regions are formed by cuts in the impendence matchinglayer.
 13. The device of claim 11 wherein the low-permittivity regionsimpede displacement currents.
 14. The device of claim 11 wherein thelow-permittivity regions act as series elements between thehigh-permittivity regions.
 15. The device of claim 11 wherein thelow-permittivity regions cause the device to present a smaller load thanwithout the low-permittivity regions.
 16. The device of claim 11 whereinthe impedance matching layer comprises a plurality of slots extendingtherethrough, said slots also extending partially through thepiezoelectric transducer.